a. Field of the Invention
This invention relates generally to multiplex stereophonic receivers, and more particularly it is directed to an improvement in an averaging demodulator circuitry of a receiver designed specifically for the reception of both multiplex stereophonic and monophonic programs in the frequency modulation, i.e. FM broadcast band.
B. Description of Prior Art
It is well known that, for reception of multiplex stereophonic broadcast program by these types of receivers, and incoming stereophonic composite signal must be separated into an audio signal representing a left hand channel (hereinafter referred to as the "L signal") and another audio signal representing a right hand channel (hereinafter referred to as the "R signal"), the L and R signals being stereophonically related with each other. Heretofore, there have been known two schemes for converting the stereophonic composite signal into the separate L and R signals, one using a matrix circuit and the other using a switching circuit. For reasons of relative simplicity in construction and a higher degree of separation obtained, the latter scheme is now generally preferred. However, according to known circuit arrangements based on this latter scheme, which usually incorporate diodes or transistors, distortion occurs inevitably as a result of the switching operation; and moreover, the output levels of their de-emphasis filters during the stereophonic reception are usually about 6 decibels lower than those levels observed during the monophonic reception.
Also known, as disclosed in, for example, U.S. Pat. No. 3,301,959 to R. Jenkins, is a circuit arrangement comprising: a switching demodulator provided posteriorly to a single power amplifier in order to omit one more power amplifier which ordinarily is required to perform power-amplication of both the L-channel and R-channel audio signals assigned to drive the speakers; capacitors provided on the output lines of the switching demodulator for sample-holding the chopped outputs delivered from the demodulator and thereby producing envelope-detected continuous audio signals; speakers for converting the resulting audio signals into sounds; and negative feedback paths for feeding the continuous audio signals back to the power amplifier to stabilize the operation of this power amplifier. However, the audio sounds through envelope detection lack high fidelity to the original input signals.